The present invention relates to a process for the reclamation of waste paper.
Waste paper from salvage is an important source of raw material for the paper industry, especially in the manufacture of packing papers and cardboards. As the rate of utilization of recovered fibers in packing products is already very high, other outlets for waste paper have also been considered, such as the manufacture of printing/writing paper or that of tissue paper. In this case, it is important to have an initial pulp with a sufficient degree of whiteness. This degree of whiteness can be obtained by resorting to the so-called de-inking and/or bleaching techniques which comprise especially a disintegration stage for waste paper in a pulper and an ink elimination stage by washing or flotation. Other stages, such as, for example, vat storage, elimination of heavy and light impurities, breaking down of agglomerates, elimination of inks by washing or flotation, are in general also included in the treatment cycle.
During the disintegration stage, waste paper is put into the pulper at the same time as an aqueous alkaline solution. The latter can contain an alkaline hydroxide, such as sodium hydroxide, a mixture of alkali metal hydroxide, and, more particularly, sodium hydroxide, with hydrogen peroxide or again an alkali metal peroxide, such as sodium peroxide.
In addition to these compounds, the aqueous alkaline solution can also contain other additives, such as sodium silicate, possibly mixed with derivatives, such as collectors or foaming agents in the case of de-inking by flotation or mixed with dispersants in the case of de-inking by washing. The presence of silicate allows a very marked improvement to be obtained in the results achieved especially as regards the dispersion of inks. In addition, when the alkaline solution contains a peroxide, the silicate has the advantage of stabilizing, at least partially, the peroxide. Aqueous solutions based on hydrogen peroxide, sodium silicate and sodium hydroxide are most often used.
In these conventional processes, the quantities of silicate to be employed are, however, very large. Moreover, silicate is sometimes troublesome since it produces precipitates which can choke the wires of paper machines, the pipes and the flotation cells. Also, when working in the presence of silicates, deposits are observed in the de-inking installation which can absorb the inks. In addition, when the pulp is used in the manufacture of tissue paper, papers with a rough feel are obtained which are not suitable for use. Finally, when working in the presence of a peroxide compound, such as hydrogen peroxide or sodium peroxide, in spite of the presence of silicate, a rather large consumption of peroxide compound is observed.